Stop Gate

This game is taken from the book On Numbers and Games by John Conway (Academic Press, 1976) and is attributed to Goran Andersson.

What you need

• Checkerboard and a set of dominoes
• Checkerboard and strips of paper
• Graph paper and a pencil
• A grid of dots and a pencil

Stop Gate with a Checkerboard and Dominoes

The dots on the dominoes are irrelevant for this game, so you can turn them upside down. If you don't have dominoes, you can use domino-sized strips of paper or two checkers (or poker chips, or unifix cubes) placed side by side. (In the pictures that follow, the dominoes are shown in blue, instead of their traditional black, so they can be distinguished from the black squares on the checkerboard.)

To play

• One player starts by placing a domino over two adjacent squares on the checkerboard. The domino can go in an up-down direction or left-right but not diagonally. Whatever direction the player chooses will be the direction in which that player always places dominoes during the game. In the example below, the first player has chosen the up-down direction.

• The second player places a domino over two adjacent squares in the direction not chosen by the first player. The domino may not be in any squares that are already occupied by a domino. This is a move the second player could choose in this game.

• The first player places another domino in the correct direction.

• And then it is the second player's turn.
• The game ends when one of the players is unable to find a place to put a domino.
• The the player who was unable to play loses.
• This is how the game that was started above might look by the time it is finished. (Who won?)

Stop Gate with paper and pencil

You can use graph paper of any size to play stop gate. You don't have to limit yourself to the size of the checkerboard. You don't need dominoes, either. Draw lines with a pencil to show where the dominoes go. A game of Stop-Gate on graph paper might start out looking like this.

If you don't have graph paper and you don't have a checkerboard, you can still play if you have a blank sheet of paper on which you have drawn a grid of dots. (The grid can be any size.)

Here is how a game on a grid of dots might look after a few turns.

Notice that only one line can touch a dot.

This is okay:

Two lines touch a dot here. It is not legal!

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